Background:To study in detail levels and patterns of physical activity in preschool children and the effect of gender and body mass index on this activity.Methods:Two hundred and fifty-three children aged 5 years participating in the Copenhagen Prospective Studies on Asthma in Childhood (COPSAC2000) wore an accelerometer day and night over a 4-weekperiod. The main outcome measure was level of physical activity using the raw data.A secondary measure was time spent in moderate to vigorous physical activity. A Fourier series analysis was applied to study in detail patterns over time.Results:Activity profiles throughout the year were unique for each sex with boys being overall more active than girls except for winter months. Preschool children also showed distinct patterns of physical activity during weekdays compared to weekends, and were most active in weekdays. Preschool children in the highest tertile of body mass index had a flat yearly activity profile and tended to be less active compared to those in the lowest tertile.Conclusion:Preschool children showed significant gender differences in physical activity with distinct patterns throughout the year as well as between weekdays and weekends. A high body mass index tended to be associated with lower levels of physical activity.Pediatric Research (2013); doi:10.1038/pr.2013.99.